This invention relates to audio amplifiers.
It is well accepted that with electrically amplified guitar and similar music, the characteristics of the amplification circuitry and loudspeakers play a fundamental role in attaining the desired sound. Since the 1960's, valve-based guitar amplifiers driving traditionally constructed guitar-type loudspeaker cabinets, have become associated with a characteristic and highly regarded sound quality. These electrical valve amplifiers remain in high demand in particular sectors of the music industry, despite the fact that valve technology is regarded as obsolete in many other electrical fields.
Numerous attempts have been made to emulate, using solid-state circuitry, the characteristic sound of a valve amplifier and traditional loudspeaker cabinet. Efforts have been made to produce line level audio signals which when reproduced through headphones, recording equipment or full range linear loudspeakers, simulate the sound achieved with a traditional loudspeaker cabinet. Thus, solid state guitar amplifiers are available with distortion effects deliberately introduced to imitate the distortion inherent in valve amplifiers. Loudspeaker emulaters have been proposed (see for example WO 88/00410) which provide a valve amplifier with a reactive load impedance equivalent to that of a traditional loudspeaker cabinet and which filter the audio signal in simulation of the frequency response of a loudspeaker.
This invention is directed to solid state amplifiers which can be used either to drive traditional loudspeaker cabinets or, with appropriate loudspeaker emulation, to provide line level signals. It is an object of this invention to provide an improved solid-state amplifier or amplifier stage which reproduces more faithfully the output of a valve amplifier driving a traditional loudspeaker cabinet.
It has been found by the present inventor that when driving either a traditional loudspeaker cabinet or a loudspeaker emulator, it is not enough for a solid state amplifier to simulate the distortion and related characteristics of a valve amplifier. It is now recognised that the complex load impedance provided by a traditional loudspeaker cabinet has--by virtue of the comparable output impedance of the valve amplifier--a marked effect upon the effective gain of the amplifier. If the valve amplifer is replaced by a solid state amplifier which has the identical "bench" characteristics but a low output impedance, this frequency-dependent gain effect is lost. There is accordingly a degradation in sound quality irrespective of whether the amplifier is used to drive a traditional loudspeaker cabinet or an appropriate emulation.
An approach to this problem is to provide a solid state amplifier with an artificially high output impedance. There are, however, significant problems with this approach. For example, a resistive load connected in series with the amplifier output will dissipate unacceptable high power levels. Alternatives exist but it is still desirable to retain the low output impedance normally associated with solid state technology.